1. Field of the Invention
The present invention relates to a nonreciprocal circuit device such as an isolator and a circulator for use in a high frequency bandwidth including a microwave band, a nonreciprocal circuit constituted together with the nonreciprocal circuit device, and a communication device using these components.
2. Description of the Related Art
Hitherto, nonreciprocal circuit devices such as a lumped constant isolator and a lumped constant circulator have been used for communication devices taking advantage of the characteristic that the attenuation of the signal is extremely small in the transmission direction, and extremely large in the reverse direction.
FIG. 7 is an exploded perspective view of a prior art isolator, FIGS. 8A and 8B are a top view and a cross-sectional view of its internal structure, and FIG. 9 is an equivalent circuit diagram, respectively.
As shown in FIGS. 7, 8A and 8B, in this isolator, a magnetic assembly 5 comprising central conductors 51, 52 and 53 and a ferrite 54, a permanent magnet 3 and a resin case 7 are disposed in a magnetic closed circuit mainly comprising a top yoke 2 and a bottom yoke 8. Port sections P1 and P2 of the central conductors 51 and 52 are connected to input/output terminals 71 and 72 and matching capacitors C1 and C2 formed in the resin case 7, a port section P3 of the central conductor 53 is connected to a matching capacitor C3 and a terminating resistor R, and each one end of the capacitors C1, C2 and C3 and one end of the terminating resistor R are connected to a ground terminal 73.
In an equivalent circuit shown in FIG. 9, the ferrite is expressed as a disc, the DC magnetic field as H, and the central conductors 51, 52 and 53 as an equivalent inductor L, respectively.
In a general communication device, an amplifier used in the circuit surely generates a certain distortion, causing the unwanted radiation such as second and third harmonic components of the fundamental wave. Since the unwanted radiation of the communication device causes an abnormal operation and radio interference of a power amplifier, and thus, the rules and standards are specified therefor in advance, and the level of the unwanted radiation must be below the specified value. In order to prevent the unwanted radiation, it is effective to use an amplifier with excellent linearity, but it is expensive, and a method in which a filter or the like is provided in place thereof to attenuate the unwanted frequency components is generally adopted. However, the use of such a filter is costly and the size of the communication device is increased, and losses by the filter are generated.
On the other hand, in the communication device, an isolator and a circulator are used for the stable operation and protection of an amplifier in the circuit, and in particular, the isolator and the lumped constant circulator have the characteristic of the band pass filter in the transmission direction characteristic that the signal is attenuated even in the transmission direction in the frequency band away from the pass band. However, in the nonreciprocal circuit device having only a prior art basic structure shown in FIGS. 7 to 9, no sufficient attenuation characteristic can be obtained in the unwanted frequency band.
A nonreciprocal circuit device capable of obtaining a large attenuation in the frequency band of the unwanted radiation such as mainly second and third harmonic components of the fundamental wave is shown in the Japanese Unexamined Patent Application Publication No. 10-93308, corresponding to U.S. Pat. No. 6,020,793. FIG. 10 is an exploded perspective view of the isolator, FIGS. 11A and 11B are a top view and a cross sectional view of its internal structure, and FIG. 12 is an equivalent circuit diagram, respectively as its constitution.
Difference of a device in FIGS. 10 to 11B from the prior art device in FIGS. 7 to 8B is that the inductor Lf for the band pass filter is provided. This inductor Lf is connected between the port section P1 of the central conductor 51, the matching capacitor C1 and the input/output terminal 71.
As shown in the equivalent circuit in FIG. 12, a band pass filter is constituted by this capacitor Cf and the inductor Lf by connecting the capacitor Cf to the input/output terminal 71 in series.
Thus, the whole communication device can be reduced in size compared with a case in which a single filter is installed outside by providing at least an inductor for the filter to attenuate the unwanted frequency band in the nonreciprocal circuit device. However, at the request for further reduction in size of the recent mobile communication equipment, the nonreciprocal circuit device itself provided with such an inductor for filter is also requested to be reduced in size. The inductor for filter must also be reduced in size. However, if the inductor formed in solenoid shape is reduced in size, its inductance is reduced, and the attenuation with second and third harmonic components of the fundamental wave is reduced. A structure in which a solenoid is formed within a magnetic member can be reasonably devised to reduce in size the solenoid-shaped inductor without reducing its inductance; however, in such a structure, there are problems that a magnetic member is newly required, its manufacture is not easy, and the cost is increased.
Accordingly, it is an object of the present invention to provide a nonreciprocal circuit device with at least an inductor for filter built therein which is compact and capable of obtaining a large attenuation in the specified frequency band without increasing the cost, a nonreciprocal circuit constituted together with the nonreciprocal circuit device, and a communication device using it.
In the nonreciprocal circuit device of the present invention comprising a plurality of central conductors overlappingly intersecting with each other and disposed on a magnetic member for receiving a DC magnetic field, a solenoid-shaped inductor is connected between at least one port section of the central conductors and a signal input/output terminal, and the inductor is disposed so that the direction of the magnetic flux generated by the inductor and passing through the magnetic member is substantially perpendicular to the direction of the DC magnetic field.
As shown in FIGS. 10 to 11B, in a prior art nonreciprocal circuit device, the magnetic flux generated by the inductor passes through the magnetic member (ferrite) in the direction parallel to the DC magnetic field; however, since the relative magnetic permeability in the direction parallel to the DC magnetic field of the magnetic member is 1, the inductor works only as the hollow core solenoid-shaped inductor. However, the relative magnetic permeability in the direction perpendicular to the DC magnetic field of the magnetic member is greater than 1, a substance high in relative magnetic permeability is interposed in the magnetic path of the inductor by the structure of the present invention, and the inductance of the inductor is increased. Thus, the inductor to obtain the specified inductance is reduced in size, and the whole nonreciprocal circuit device is reduced in size.
A nonreciprocal circuit of the present invention comprises the nonreciprocal circuit device and a capacitor connected to its inductor in series, and a band pass filter is formed of the capacitor and the inductor. The spurious such as second and third harmonic components of the fundamental wave is considerably attenuated thereby.
The nonreciprocal circuit of the present invention forms a low pass filter comprising capacitors connected between both ends of the inductor of the nonreciprocal circuit device and a ground, and the inductor. Unwanted frequency components are considerably attenuated thereby.
A communication device of the present invention is formed using the nonreciprocal circuit device or nonreciprocal circuit for, for example, a transmitting/receiving circuit of an antenna sharing circuit. A communication device compact and excellent in sprious characteristic is obtained.
According to the present invention, a substance high in relative magnetic permeability is interposed in the magnetic path of the inductor in the invention, the inductance of the inductor is increased, the inductor to obtain the specified inductance can be reduced in size, and the whole nonreciprocal circuit device can be reduced in size.
According to the present invention, the characteristic with both the nonreciprocal circuit characteristic and the band pass filter characteristic is obtained, the unwanted frequency component can be suppressed without separately providing any filter, and a device using this nonreciprocal circuit device can be reduced in size.
According to the present invention, the device can be reduced in size while suppressing the unwanted radiation from the device.